In wireless communication systems, data to be communicated is typically transmitted in bursts on a carrier whose characteristics vary over time. In other words, a first burst of data might be transmitted over the carrier while the carrier has very good performance that allows the first burst of data to be received correctly, while, as a second burst of data is transmitted on the carrier, the performance of the carrier might have worsened such that the second burst of data is not received correctly. This problem can be explained by the fact that carriers include multiple paths; therefore, even a small movement by either a transmitter or a receiver can affect whether these multiple paths add constructively or destructively.
If a rate of change of the performance of a carrier is relatively great in comparison to a data rate on the carrier, the problem of varying carrier performance can be solved using coding and interleaving, in which carrier performance variations are averaged so that the carrier's performance depends on average carrier conditions rather than on worst-case carrier conditions. However, if the carrier's performance varies relatively slowly and/or if the data rate is relatively great, this approach is not feasible because the number of symbols needed in an interleaver is too large. In such situations, an entire packet could be received during a period in which the carrier's performance is poor.
Multi-path carriers are frequency-selective; therefore, if performance of a first carrier having a first frequency is poor, performance of a second carrier having a second frequency is often better, especially if the second frequency is not too close to the first frequency. The coherence bandwidth is a measure of how far apart the two frequencies must be in order for the two carriers to be uncorrelated.
Reference is now made to FIG. 1, wherein there is shown a graph representing an exemplary indoor bandwidth operating at 2.45 GHz as a function of frequency. Graph 100 shows frequency in MegaHertz (MHz) on an x-axis and on a y-axis, signal strength is plotted in decibels (dB) (i.e., 20 log [abs(H(f))]. A bandwidth 102 is shifted on the x-axis so that 0 Hz corresponds to the carrier being used. The bandwidth 102 has good performance near 0 Hz and 10-15 MHz, whereas it is about 10 dB worse near 23 MHz, and has its worst performance near 6 MHz. The bandwidth of FIG. 1 has a coherence bandwidth of about 10 MHz.
One way of communicating over a frequency-selective carrier is by means of frequency hopping (FH), which is used, for example, in the BLUETOOTH wireless technology system. See, e.g., J. C. Haartsen, “The Bluetooth radio system,” IEEE Personal Communications, Vol. 7, No. 1, February 2000. In the BLUETOOTH wireless technology system, which is an ad-hoc system that operates in the unlicenced Industrial Scientific and Medical (ISM) band at 2.45 GHz, one of the reasons for employing FH over 79 1-MHz-wide carriers is to avoid transmitting on a single carrier that could be strongly attenuated for a long time period due to multi-path fading. Another reason for using FH is to have a system that is robust to interference from other users as well as from other impairments.
Frequency hopping is a way of averaging quality of the total available bandwidth, and, in situations in which the carrier performance changes rapidly, FH often provides best-case real-world performance. However, in situations in which a portion of the bandwidth changes slowly, it would be desirable to further improve performance. For example, if a part of the bandwidth is disturbed by an almost static interferer, this part of the bandwidth should typically be avoided. A static interferer could, for example, be a turned-on microwave oven, since many microwave ovens use part of the ISM band.
Carriers that are operating in a part of the bandwidth that is disturbed by almost-static interferer(s) should be avoided. A procedure for selecting suitable carriers that are not affected by the almost-static interferer(s) would be desirable. There is accordingly a need for a method and system for carrier selection in response to worsening carrier performance when the carriers performance varies relatively slowly and/or the data rate is relatively great.